Helicopter rotor designs are increasingly utilizing a flexible structural member, commonly termed a "flexbeam" or "flexbeam connector", for retention of a helicopter rotor blade to a torque drive hub member. Basic operational constraints of rotary wing flight impose substantial functional complexity upon the rotor flexbeam necessitated by the need to control accurately multi-directional displacement of the rotor blades, i.e., flapwise and edgewise bending, and torsional or pitch change motions. As such, these configurations are termed "Bearingless Rotors" inasmuch as they replace antiquated bearing element rotors which accommodate motion by hinge or journal type bearings at the rotor blade root end. The flexbeam connector, which is typically comprised of fiber reinforced resin matrix materials, reduces the weight, complexity, and maintenance of the rotor assembly while, furthermore, improving the reliability and damage tolerance thereof.
Bearingless rotors of the varieties described in U.S. Pat. Nos. 4,244,677, and 5,092,738 typically include a torque tube enveloping each of the flexbeam connectors for imparting pitch motion to the rotor blades. The torque tube rigidly mounts outboard to the root end of the rotor blade and articulately mounts inboard to the upper and lower surfaces of the flexbeam connector. The articulate mount is effected by upper and lower snubber bearings which perform the functions of centering the torque tube relative to the flexbeam connector for pitch change and flapping motion, accommodating lead-lag motion between the torque tube and flexbeam connector and transferring pitch control and other loads therebetween. The snubber bearings are internally mounted within the torque tube and interposed between surfaces of the flexbeam connector and upper and lower portions of the torque tube. The snubber bearings are typically comprised of a plurality of spherical and flat elastomeric laminates, which spherical laminates accommodate pitch change and flapwise bending motion and which flat laminates permit a small degree of radial and a larger degree of edgewise motion. The flat laminates are, furthermore, comprised of high loss elastomer material for providing edgewise or lead-lag vibration damping.
Each snubber bearing includes inner and outer race portions which, respectively, correspond to the flexbeam and torque tube mounting locations. The inner race of each snubber bearing is typically mounted to a retainer which is affixed to the upper and lower surfaces of the flexbeam connector. Insofar as such location is generally inaccessible for mounting purposes, the inner race commonly includes radially extending tabs for slideably engaging slots or channels formed in the retainer. Such sliding engagement facilitates ease of assembly and replacement. The accessibility of the outer race portions of the snubber bearings permits attachment by more conventional means e.g., a bolted attachment.
Since the inboard section of the flexbeam connector is exposed to high levels of axial, bending and torsional stress, it is preferable to bond the retainers to the flexbeam connectors to avoid stress inducing apertures therein such as those necessary for a bolted attachment. Furthermore, the retainers are typically formed from metal stock to facilitate machining of the channels which accepts the inner race tabs of the snubber bearings.
While such bonded metal retainers are satisfactory for lightly loaded structural applications, e.g., helicopter tail rotors, their use in highly loaded applications, e.g., main rotors, is more problematic. The effects of flexbeam motion and increased loads associated with a more demanding operational environment generate high shear stresses within the bondline between the retainer and the flexbeam connector. For example, it will be appreciated that the size and mass of the snubber bearings are proportionally larger in helicopter main rotor applications and, consequently, produce higher centrifugal loads. Inasmuch as these centrifugal loads are reacted in shear across the bondline, high shear stresses are developed therein which can result in bondline failure. Furthermore, high strain levels associated with the large flapwise and torsional motion of a main rotor flexbeam connector induces large shear loading within the bondline which exacerbates the problem of bondline failure. Other sources of bondline failure relate to the high axial strain generated by the high blade-induced centrifugal loads acting on the flexbeam connector. Insofar as the modulus of the metal-formed retainer is substantially higher than the composite-formed flexbeam connector, bondline shear stresses are developed due to the differences in material properties. That is, the composite-formed flexbeam connector elongates at a significantly higher rate than the metal retainer under the same applied load, which elongation must be accommodated in the bondline area.
Should bondline failure occur, the retainer and inner race of the snubber bearing will shift under the influence of the various imposed loads. As a result, the dislocated snubber bearing may adversely affect the degree of pitch control available, introduce undesired pitch control inputs, change the natural flapwise and edgewise frequencies of the rotor blade, and/or impair the damping efficacy of the snubber bearing.
Byrnes et al. U.S. Pat. No. 5,092,738 describes a mounting assembly for securing a retainer of the type described above which employs composite wrap members for preventing motion of the retainer in the event of a bondline failure. While the wrap members prevent lateral motion of the retainer, such retention is ineffective for preventing spanwise or axial displacement of the snubber bearings.
A need, therefore, exists for providing an improved mounting assembly for securing a snubber bearing to a bearingless rotor assembly which minimizes bondline stresses and, furthermore, provides redundant retention.